1. Field of the Invention
The invention relates to a transmission ratio variable device.
2. Description of the Related Art
In related art, various transmission mechanisms using gears are known. A transmission mechanism, which can obtain a large speed reducing ratio with a small number of components and have large transmission capacity, is desired. As the transmission mechanism, there have been proposed a transmission ratio variable device in which rotation based motor driving is added to the rotation of an input shaft based on a steering operation, using a differential mechanism so as to transmit resultant rotation to an output shaft, thereby changing the rotation transmission ratio (steering gear ratio) between the input and output shafts, and a vehicle steering system including the transmission ratio variable device (for example, refer to Japanese Patent Application Publication No. 2006-82718).
In a speed-reducing mechanism of the transmission ratio variable device, a nutation gear mechanism is provided as a differential mechanism. The nutation gear mechanism includes a first gear that rotates together with an input shaft, a fourth gear that rotates together with an output shaft, and a nutation gear. The nutation gear includes a second gear that meshes with the first gear, and a third gear that meshes with the fourth gear, and the nutation gear rotates around an axis that inclines with respect to the axes of the first and fourth gears. The nutation gear is configured so as to rotate according to differences in the number of teeth between the first gear and the second gear that mesh with each other and between the fourth gear and the third gear that mesh with each other, while oscillating in the direction of the axis of the input shaft via a bearing. The nutation gear reduces the speed of the rotation input from the input shaft, and transmits the rotation, whose speed has been reduced, to the output shaft, and the nutation gear is rotated by driving means to change the rotation transmission ratio between the input shaft and the output shaft.
In the above-described transmission ratio variable device, for example, when a coupling portion between the input shaft or the output shaft and a gear of the speed reducer is formed by cylindrical surfaces, knurling press-fitting may be used as a fastening method due to constraints imposed on the physical size of the device in terms of mounting. A knurled shaft is press fitted into a hole. However, the knurling press-fitting is performed while an inner peripheral surface of a cylindrical component is shaved off. Thus, there is a possibility that chips may be generated and the generated chips may hinder the rotation of a bearing or a gear at the speed reducer-side, as foreign matter.